But specific genes for the mental health conditions have yet to be pinpointed.

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TUESDAY, Aug. 14, 2012 (HealthDay
News) — Two new large-scale studies searching for the
genetic links to a couple of relatively common psychiatric
conditions show how difficult it can be to decipher the human
genome's role in disease.

The research, the first genome-wide studies looking at the potential
association of particular genes with obsessive-compulsive disorder (OCD) and Tourette
syndrome, was published in two reports in the Aug. 14 issue of
Molecular Psychiatry.

While the studies failed to identify particular genes responsible for
either of these conditions, it contributed important new clues.

"The studies suggest there are probably lots of different genes
of small effect that play a role, or, perhaps, there is a rare single
gene," said Dr. Francis McMahon, chief of the human genetics branch within
the intramural research program at the U.S. National Institute of Mental
Health.

While the research did not identify a particular gene or combination
of genes responsible for either Tourette syndrome or OCD, it does contribute to
the understanding of the genetic architecture of both conditions, McMahon said.
"Unfortunately for those who suffer from these conditions, that means
there's not much hope for a quick drug target to be developed from this
research," he said.

Obsessive-compulsive
disorder is an anxiety condition characterized by repetitive,
recurrent thoughts and compulsions that disrupt people's lives. Tourette
syndrome is a chronic, inherited disorder of the nervous system, characterized
by unwanted movements and noises called "tics." It often begins in
childhood and can be associated with OCD or attention-deficit/hyperactivity
disorder (ADHD).

The conditions often run in families, which makes them prime targets
for genetic research. Previous studies that compared affected and unaffected
people were not large enough to detect specific genes or areas of the genome
responsible for increasing a person's risk for either or both
conditions.

Genome-wide studies, known as GWAS, look at common genetic variants
among different people to find associations with diseases or traits. They
compare the DNA of those with a condition to others without it.

The genetic details are analyzed, looking for gene variants that show
up more frequently in people with the disease than in those without it. Variations
in the individual make-up of a genome are identified by noting areas where
there are changes and variations that could heighten the risk of disease.

For these studies, the International OCD Foundation Genetic
Collaborative, which includes more than 20 research groups in nine countries,
analyzed about 480,000 gene variants called SNPs (single-nucleotide
polymorphisms) in 1,465 people with OCD, more than 5,500 without OCD, and in
others including both parents of someone with the condition.

The Tourette Syndrome Association International Consortium for
Genetics and the Tourette Syndrome GWAS Consortium, which includes 22 groups in
seven nations, analyzed 484,000 SNPs in about 1,500 individuals against 5,250
people without Tourette syndrome.

McMahon explained that the purpose of the GWAS approach is to look
across the whole genome for common forms of variation, looking for genetic
differences present in a quarter, a third, or half the population. "That's
where the statistical power lies," he noted.

The researchers identified a signal within a gene in people with
Tourette syndrome. That gene encodes a protein in the cerebellum, an area of
the brain involved in motor control. They also found that a strong signal close
to another gene, active in the brain during childhood and adolescence, was
associated with those with OCD, which often emerges during that particular time
of life. But none of the discoveries were closely enough associated to identify
risk factors.

Dr. James Leckman, a co-author of the studies and a distinguished
professor of child and adolescent psychiatry at Yale University, explained that
genetic analysis is complicated by the fact that while some people have just
Tourette syndrome or only OCD, a percentage has both, and others also have
ADHD. Some have very resistant symptoms and others are treated effectively,
usually with cognitive behavioral therapy.

"GWAS studies lump everybody together but we need to tease out
the differences," Leckman said. "I'm not sure the GWAS strategy will
get us where we need to go."

Three of every 1,000 children have Tourette syndrome, according to the
U.S. Centers for Disease Control and Prevention. The American Psychiatric
Association says more than 4 million people have OCD.

"These are chronic conditions; people with OCD and Tourette's
deal with it every day. Both, at their worst, are debilitating, and the anxiety is always
present," said Alies Muskin, executive director of the Anxiety and Depression
Association of America, in Silver Spring, Md.

Tourette syndrome and OCD can be treated with cognitive behavioral and
exposure therapy, which train people to deal with their anxiety, Muskin
said.

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